Conjugated dienes, particularly isoprene, are basic building blocks of natural and synthetic rubber. As a matter of fact, the C.sub.5 structure of isoprene is the basic unit of many natural products. Natural products whose structures are composed of isoprene units are referred to as isoprenoids. While isoprene itself has not been encountered in nature, the biogenetic path of synthesis of the terpenes involves the incorporation of the C.sub.5 repeating unit into a large variety of open and ring structures. The isoprene molecule is, therefore, of considerable importance in the synthesis of natural products. .
Attempts to synthesize natural products from isoprene normally involve the functionalization of isoprene. Successful functionalizations of isoprene have involved the addition of anhydrous HCl to isoprene yielding a mixture of prenyl chloride and isoprenyl chloride, the direct halogenation of isoprene yielding mixtures of dihalomethylbutenes, and reaction with tosyl chloride to give positional and cis/trans isomer mixtures of C.sub.5 chlorosulfones. The utility of the halogenated isoprenes, however, is quite limited due to the difficulty of replacing the halogen substituents selectively with other functional groups. In view of the considerable interest in the synthesis of natural products and other materials employing the isoprenoid unit, there is a continuing effort in the area of isoprene functionalization.
It has been found that isoprene and other conjugated dienes can be regiospecifically difunctionalized according to the process disclosed and claimed herein. The novel compounds, because of the presence of the nitro group are readily amenable to the transformation of the nitro group to other functional groups, e.g., amino, carbonyl, cyano, oximes, etc. The presence of the alkoxy carbonyl (ester) moiety also provides another reactive site. One advantage is that the resultant products of the instant process are substantially in the transconfiguration, with regard to the disposition of the nitro and ester groups, as opposed to cis and trans mixtures, which are generally obtained when isoprene is bishalo substituted. Another important advantage is that the nitro group attaches itself practically exclusively at the one position of the conjugated diene carbon skelton thereby obviating the need to separate any positional isomeric species.
The isoprenoid structure also forms the basis of one of the starting materials in vitamin A synthesis, i.e., .gamma.-acetoxytiglic aldehyde or derivatives thereof. There is a continuing effort to find new and economical ways to prepare this important starting material. (See, for example, Freyschlog et al., U.S. Pat. Nos. 3,347,930; 3,760,004; Himmele et al.; U.S. Pat. Nos. 3,661,980; 3,732,287; and 3,781,337). The instant invention provides a simple and economical way to obtain .gamma.-acetoxytiglic aldehyde.
A surprising feature is that the regiospecific difunctionalization proceeds with negligible polymerization of the conjugated diene starting material. This is unexpected in view of the ready polymerizability of conjugated dienes in acidic solution and in the presence of radical initiators, such as nitrous oxides, which cannot be excluded whenever nitric acid is employed.
Although the foregoing discussion has been directed to isoprene, it is equally applicable to conjugated dienes in general.